Incandescent lamps are commonly operated without current inrush limiters and, as a result of the current inrush with full voltage supplied to a cold filament, lamp life is generally considerably shortened. It has been generally recognized that, as the cold resistance tungsten filament is only approximately 1/20 to 1/10 of the hot resistance, the inrush current is about 10 to 20 times the normal operating current. This current inrush causes severe local overheating at high resistance spots in the filament. These spots rise to a temperature well in excess of the steady-state burning temperature of the filament. This local overheating causes more rapid evaporation of the filament in the area of these spots and the resistance of the spots is thus further raised. This process continues and is accelerated by each starting current surge, and eventually leads to the failure of a filament as a result of localized melting. To slow the process, as is well known in the art, a temperature-sensitive resistor having a negative temperature coefficient of electrical resistance may be connected in series with an incandescent lamp filament thereby greatly increasing lamp life by acting as a current inrush limiter.
Vanadium oxide has been suggested for externally-heated, temperature-sensitive resistors. In British Pat. No. 1,168,107 issued to Hitachi Limited and published Oct. 22, 1969 is described the method for producing a ceramic comprising of crystals of VO.sub.2 suspended in a semiconductive oxide flux. The properties of vanadium-oxide ceramics are also described by Futaki in Vol. 4, No. 1, Jan., 1965 of the Japanese Journal Of Applied Physics, pages 28-41. Both the aforesaid British Patent and the Futaki Publication describe a method for making VO.sub.2 current inrush limiters. Their method of making the VO.sub.2 is by reduction of V.sub.2 O.sub.5 to VO.sub.2.
It has been found that the preparation of the VO.sub.2 current inrush limiters by reduction or oxidation of higher or lower oxides, respectively, is difficult, and requires very carefully controlled conditions. Furthermore, VO.sub.2 in the required powdered form is susceptible to oxidation which presents problems in storage and handling.